# Evidence for gene flow from the Gulf of Mexico to the Atlantic Ocean in bonnethead sharks (Sphyrna tiburo)

## TL;DR

Bonnethead sharks can migrate between the Gulf of Mexico and the Atlantic Ocean, helping maintain genetic diversity despite their small size.

## Contribution

This study provides evidence of long-distance migration and unidirectional gene flow in bonnethead sharks, challenging expectations for small coastal sharks.

## Key findings

- Bonnethead sharks show genetic connectivity between Gulf of Mexico and Atlantic Ocean populations.
- Gene flow is unidirectional, from Tampa Bay (Gulf) to Biscayne Bay (Atlantic).
- Migrations of over 300 miles suggest bonnetheads can sustain genetic diversity through long-distance mating.

## Abstract

Gene flow is important for maintaining the genetic diversity required for adaptation to environmental disturbances, though gene flow may be limited by site fidelity in small coastal sharks. Bonnethead sharks (Sphyrna tiburo)—a small coastal hammerhead species—demonstrate site fidelity, as females are philopatric while males migrate to mediate gene flow. Consequently, bonnetheads demonstrate population divergence with distance, and Atlantic populations are genetically distinct from those of the Gulf of Mexico. Indeed, Florida forms a vicariant zone between these two bodies of water for many marine species, including some sharks. However, while bonnetheads are expected to have limited dispersal, the extent and rate of bonnethead migration remain uncertain. Thus, we aimed to determine their dispersal capacity by evaluating connectivity between disparate populations from the Gulf of Mexico and Atlantic Ocean. Using 10,733 SNPs derived from 2bRAD sequences, we evaluated genetic connectivity between Tampa Bay on the Gulf Coast of Florida and Biscayne Bay on the Atlantic coast of Florida. While standard analyses of genetic structure revealed slight but significant differentiation between Tampa Bay and Biscayne Bay populations, demographic history inference based on the site frequency spectrum favored a model without divergence. However, we also estimate that if population divergence occurred, it would have been recent (between 1500 and 4500 years ago), with continuous unidirectional gene flow from Tampa Bay to Biscayne Bay. Our findings support the hypothesis that bonnetheads can migrate over relatively large distances (>300 miles) to find mates. Together, these results provide optimism that under proper management, a small‐bodied globally endangered shark can undergo long migrations to sustain genetic diversity.

Genomic data from bonnethead sharks reveals connectivity between the Gulf and Atlantic coasts of Florida, a zone that forms a genetic barrier for some larger shark species. Our demographic model also revealed rapid population growth and gene flow from the Gulf Coast to the Atlantic Coast, suggesting that sharks may migrate south for mating. These findings provide novel insight into the migratory behavior of small‐bodied sharks and optimistic implications for the conservation of this globally endangered species under proper management.